Polymer compositions that contain particles of a metal or cconductive carbon may be electrically conductive. Many such compositions, particularly those having a crystalline polymer base, exhibit the property that, as their temperature is raised, a substantial rise in the compositions ' electrical resistance occurs at a specific temperature or over a relatively narrow temperature range. In the case of compositions based upon crystalline polymers, this phenomenon is exhibited at or near the crystalline melting point or melting range.
Compositions that exhibit this property are said to possess a positive temperature coefficient (PTC) of resistance. The compositions themselves are frequently referred to as PTC compositions.
In recent years, PTC compositions have been usefully employed as components in self-regulating heating elements for electrical appliances having a variety of applications. In operation, at a constant applied voltage directed across a PTC heating element, the current (I=E/R) through the heater will be large at low temperature. The power (P) generated by this current (P=I.sup.2 R) is dissipated as Joule heat thereby warming the PTC composition. If the applied voltage is high enough, the temperature will continue to rise without a significant increase in resistance until the T.sub.s temperature is reached. At this point, a further increase in temperature results in a significant rise in resistance. Since the applied voltage is constant, concomitant with the increase in resistance is a large decrease in current and, therefore, power generation. In effect then, the heater is switched off.
The heat built up in the PTC composition dissipates by heating its surroundings, which might, for example, be a heating plate for a coffee pot, until its temperature drops below T.sub.s at which point the power output of the heater again rises. In actual practice, a steady state condition is attained at about the T.sub.s temperature as heat lost to the surrounding is offset by heat being generated within the PTC composition. The net effect of all this is that the power being generated by the current in the PTC composition remains relatively constant as does its heat output without resort to thermostats or a protective device such as a fuse.
To those skilled in the art, the availability of simple self-regulating heating devices that do not require expensive and/or bulky heat control devices suggests many applications for heating elements comprising PTC compositions. A particularly useful element takes the form of a thin ribbon or strip of carbon filled polymeric material having electrodes at its opposite edges, typically bundles of copper wires embedded in the polymer strip along its edges, parallel to the long axis of the strip. The two electrodes establish a potential gradient along the plane of the strip and transverse to its long axis. Therefore, a voltage can be applied across the electrodes to heat the entire strip to about its T.sub.s temperature. Such strips can be wrapped about and used to heat tubular or irregular conduits or vessels, for example, to thaw their contents, to prevent the salting out of solids in solution, etc.
As indicated supra, in the case of PTC compositions based on crystalline polymers, T.sub.s is related to the crystalline melting point or range of the polymer. Therefore, in principle at least, to achieve a heating element having a specific self-regulating temperature, it is only necessary to select a composition having a base polymer with the desired T.sub.s. However, the development of resistance heating elements employing PTC compositions has been hampered by the fact that many otherwise suitable polymers lack thermal or hydrolytic stability at elevated temperatures. One organic polymer that has been found to be very useful in the production of PTC compositions for high temperature applications is the largely head to tail polymer of vinylidene fluoride referred to as polyvinylidene fluoride (PVF.sub.2) having the repeat unit --CH.sub.2 --CF.sub.2 --CH.sub.2 --CF.sub.2 --. This crystalline polymer exhibits good thermal stability. In addition it has excellent chemical resistance and is tough, flexible and non-flamable.
The sum of these properties makes PVF.sub.2 ideally suited for use as the base polymer in a variety of PTC applications. A PVF.sub.2 composition, containing carbon black that has been crosslinked by being irradiated with gamma rays or high energy electrons has been shown to be an excellent PTC composition. The crosslinking by exposure to radiation has been demonstrated to be enhanced by the presence of the carbon. Crosslinking, as is well known to the art, allows the base polymer in a PTC composition to retain its mechanical properties above the crystalline melting point. Without crosslinking, temperature excursions above the crystalline melting point can cause the polymer to flow and, in this condition, its resistance may suddenly drop resulting in runaway heating.
It has been observed that the radiation crosslinked compositions of PVF.sub.2 and carbon black exhibiting PTC characteristics described above can be reliably employed in self-regulating heating elements when powered at relatively low voltages, i.e. at about 110 volts or below. However, it has recently been found that this composition when subjected to the higher stresses that accompany the use of higher voltages, undergoes a gradual but irreversible increase in its resistance until it reaches a level at which the heating element no longer gives off sufficient heat to be useful. Therefore, notwithstanding its many advantages, the voltage instability exhibited by PVF.sub.2 based PTC compositions have rendered them of little or no utility in many applications tht require voltages above about 110 volts.
Accordingly, in view of the shortcomings of the prior art compositions, it is an object of this invention to provide a novel PTC composition.
Another object of this invention is to provide a PTC composition having enhanced voltage stability.
Another object of this invention is to provide improved self-regulating electrical heating appliances.
The accomplishment of these and other objects will be apparent to those skilled in the art in view of the description of the invention that follows: